Adding to what Tom has already stated, the John innes drastically increases Gh alot. Coming from high tech tanks which i never tested. I had no idea of this at all and had bought a fair bit to go into my new tank but luckily speaking with Tom I'm glad he informed me. I tested it, and was gobsmacked how much it went up. Tap water gh4, John innes pushed it up to gh 13 I think. I have soft water and keep chocolate goiramis and bread them, these need very soft water and low ph. These are in a Friends tank currently and would have just been put in with my John innes capped but would have gradually suffered.
I guess most people who use the soil method never really test it, and keep fish that are adaptable to this.
I was going to choose there aquatic compost but again this does the same.
I've tested a popular pond one called 'aqua soil' which comes in a 20 litre bag, left it soaking for 4 days and tested gh and it was 5 i think and ph 6. This is what my little tank with cat litter in sits at too so I'd say if it's that your worrying about then definitely give it a go. The water seemed to clear rather quick too with it. Unlike john innes, I'm not sure as to how much ferts are in it as it doesn't say but states it will last a full growing season what ever that maybe in pond talk and is harmless to all know pond life.
I guess you could get the John innes etc, but if your wanting to keep the hardness down you'd be forever doing water changes each week I think.
Hope this helps
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